screenlocker: exit greeter gracefully when unlocking during grace time

Currently the screen locker just kills the greeter (kscreenlocker_greet)
when the screen is unlocked by the user during the grace time.
But apparently this can leave behind running screensaver processes
launched by the greeter, see the bug report.

This patch changes this to only terminate the greeter, and adds a signal
handler to the greeter to exit gracefully in this case.
The signal handler exits with return code 1, so that it is not possible
to circumvent the password input by just sending a SIGTERM. (the screen
locker restarts the greeter in case it doesn't quit with exit code 0)

CCBUG: 224200
REVIEW: 117644